A call option on a stock has a theoretical price of 8.50 according to the black Scholes model. the stock's current price is 75, the option strike price is 70 and it expire in 4 months. the risk free interest rate is 3% per annum, and the stock's volatility is 18%. calculate the implied volatility of the option 16.7%, 19.40%, 22.10%, 14.20%